In abstract algebra, a group isomorphism is a function between two groups that sets up a one-to-one correspondence between the elements of the groups in a way that respects the given group operations. If there exists an isomorphism between two groups, then the groups are called isomorphic. From the standpoint of group theory, isomorphic groups have the same properties and need not be distinguished.
Definition and notationEdit
Given two groups and a group isomorphism from to is a bijectivegroup homomorphism from to Spelled out, this means that a group isomorphism is a bijective function such that for all and in it holds that
The two groups and are isomorphic if there exists an isomorphism from one to the other. This is written:
Often shorter and simpler notations can be used. When the relevant group operations are unambiguous they are omitted and one writes:
Sometimes one can even simply write = Whether such a notation is possible without confusion or ambiguity depends on context. For example, the equals sign is not very suitable when the groups are both subgroups of the same group. See also the examples.
Conversely, given a group a set and a bijection we can make a group by defining
If = and then the bijection is an automorphism (q.v.).
Intuitively, group theorists view two isomorphic groups as follows: For every element of a group there exists an element of such that 'behaves in the same way' as (operates with other elements of the group in the same way as ). For instance, if generates then so does This implies in particular that and are in bijective correspondence. Thus, the definition of an isomorphism is quite natural.
An isomorphism of groups may equivalently be defined as an invertible group homomorphism.
In this section some notable examples of isomorphic groups are listed.
Generalizing this, for all odd is isomorphic with the direct product of and
If is an infinite cyclic group, then is isomorphic to the integers (with the addition operation). From an algebraic point of view, this means that the set of all integers (with the addition operation) is the 'only' infinite cyclic group.
Some groups can be proven to be isomorphic, relying on the axiom of choice, but the proof does not indicate how to construct a concrete isomorphism. Examples:
The number of distinct groups (up to isomorphism) of order is given by sequence A000001 in OEIS. The first few numbers are 0, 1, 1, 1 and 2 meaning that 4 is the lowest order with more than one group.
All cyclic groups of a given order are isomorphic to where denotes addition modulo
Let be a cyclic group and be the order of is then the group generated by
We will show that
Clearly, is bijective. Then
which proves that
From the definition, it follows that any isomorphism will map the identity element of to the identity element of
that it will map inverses to inverses,
and more generally, th powers to th powers,
and that the inverse map is also a group isomorphism.
The relation "being isomorphic" satisfies all the axioms of an equivalence relation. If is an isomorphism between two groups and then everything that is true about that is only related to the group structure can be translated via into a true ditto statement about and vice versa.
An isomorphism from a group to itself is called an automorphism of this group. Thus it is a bijection such that
An automorphism always maps the identity to itself. The image under an automorphism of a conjugacy class is always a conjugacy class (the same or another). The image of an element has the same order as that element.
The composition of two automorphisms is again an automorphism, and with this operation the set of all automorphisms of a group denoted by forms itself a group, the automorphism group of
For all abelian groups there is at least the automorphism that replaces the group elements by their inverses. However, in groups where all elements are equal to their inverse this is the trivial automorphism, e.g. in the Klein four-group. For that group all permutations of the three non-identity elements are automorphisms, so the automorphism group is isomorphic to and
In for a prime number one non-identity element can be replaced by any other, with corresponding changes in the other elements. The automorphism group is isomorphic to For example, for multiplying all elements of by 3, modulo 7, is an automorphism of order 6 in the automorphism group, because while lower powers do not give 1. Thus this automorphism generates There is one more automorphism with this property: multiplying all elements of by 5, modulo 7. Therefore, these two correspond to the elements 1 and 5 of in that order or conversely.
The automorphism group of is isomorphic to because only each of the two elements 1 and 5 generate so apart from the identity we can only interchange these.
The automorphism group of has order 168, as can be found as follows. All 7 non-identity elements play the same role, so we can choose which plays the role of Any of the remaining 6 can be chosen to play the role of (0,1,0). This determines which corresponds to For we can choose from 4, which determines the rest. Thus we have automorphisms. They correspond to those of the Fano plane, of which the 7 points correspond to the 7 non-identity elements. The lines connecting three points correspond to the group operation: on one line means and See also general linear group over finite fields.